Phonon Conductivity Research Articles

First-principles study of the lattice thermal conductivity of the nitride perovskite LaWN3

Recently, ${\mathrm{LaWN}}_{3}$ nitride perovskite with polar symmetry has been synthesized and has shown a large piezoelectric response. In addition to experimentally measured piezoelectricity and theoretically predicted ferroelectricity, the thermal transport properties of ${\mathrm{LaWN}}_{3}$ remains unexplored. In this work, the phonon dispersion, lattice thermal conductivity, group velocity, phonon lifetime, phase space, and Gr\"uneisen parameters of ${\mathrm{LaWN}}_{3}$ are systematically studied using first-principles calculations. The results show that the lattice thermal conductivity of ${\mathrm{LaWN}}_{3}$ is 3.33 and 3.71 W/(mK) along the $a$ and $c$ axes at 300 K, respectively, which is comparable to some typical ferroelectric oxide perovskites such as ${\mathrm{PbTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$. Further analyses imply that over 60% of the lattice thermal conductivity of ${\mathrm{LaWN}}_{3}$ is contributed by the optical phonons. It is found that there exists the coupling effect between the acoustic and low-lying optical phonons which limits the group velocity and the contribution to lattice thermal conductivity of acoustic phonons. Additionally, the four-phonon scattering is found to play an important role in suppressing the lattice thermal conducitivty, which is reduced by \ensuremath{\sim}50% compared to the values where only three-phonon scattering is considered.

Vibrational wave scattering in disordered ultra-thin film with integrated nanostructures

Abstract A theoretical model, the phase-field matching theory, has been used to investigate the localized states, their associated states, the local vibrational density of states, the coherent conductance, and the associated thermal conductivity of the perturbed ultra-thin film quasi-dimensional crystalline lattice. The defect disrupts the system’s translational symmetry in the perpendicular direction to it, which is axis Ox, and induces a localized state in its behavior that is not present in the bulk, scattering the incident elastic wave. The model was analyzed for three different cases of elastic parameters: softening, homogeneous, and hardening. The purpose is to investigate how the local dynamics can respond to changes in the microscopic environment in the perturbed domain. The analysis of the total phononic conductance spectra and the local vibrational densities states identifies distinguishing characteristics and demonstrates the sensor’s potential use in nondestroyed control.

Anomalous behavior of liquid molecules near solid nanoparticles: Novel interpretation on thermal conductivity enhancement in nanofluids

HypothesisRecently, it has been reported that anomalous improvement in the thermal conductivity of nanofluid composed of base liquids and dispersed solid nanoparticles, compared to the theoretically predicted value calculated from the particle fraction. Generally, the thermal conductivity values of gases and liquids are dominated by the mean free path of the molecules during translational motion. Herein, we present solid evidence showing the possible contribution of the vibrational behavior of liquid molecules around nanoparticles to increasing these thermal conductivities. ExperimentsThe behavior of liquid molecules in nanofluids containing SiO2 particles larger than 100 nm, which were dispersed in a 50 wt% aqueous solution of ethylene glycol, was investigated by means of small-angle neutron scattering, quasi-elastic neutron scattering, wide-angle X-ray scattering, and Raman spectroscopy. FindingsThe vibrational changes in the liquid molecules caused by the interactions between the nanoparticles and liquid molecules surrounding the nanoparticles contributed majorly to the increase in the thermal conductivity values of the SiO2 nanofluids. Because the vibration of liquid molecules is equivalent to phonon conduction in solids, the increase in thermal conductivity of the suspension due to the presence of nanoparticles was inferred to be derived from the limitation of the translational diffusion, which induces a solid-like behavior in the liquid.

Electric field-dependent phonon spectrum and heat conduction in ferroelectrics.

This article shows experimentally that an external electric field affects the velocity of the longitudinal acoustic phonons (vLA), thermal conductivity (κ), and diffusivity (D) in a bulk lead zirconium titanate-based ferroelectric. Phonon conduction dominates κ, and the observations are due to changes in the phonon dispersion, not in the phonon scattering. This gives insight into the nature of the thermal fluctuations in ferroelectrics, namely, phonons labeled ferrons that carry heat and polarization. It also opens the way for phonon-based electrically driven all-solid-state heat switches, an enabling technology for solid-state heat engines. A quantitative theoretical model combining piezoelectric strain and phonon anharmonicity explains the field dependence of vLA, κ, and D without any adjustable parameters, thus connecting thermodynamic equilibrium properties with transport properties. The effect is four times larger than previously reported effects, which were ascribed to field-dependent scattering of phonons.

Exploring the increasing behavior of thermal conductivity for high-entropy zirconates at high temperatures

Up to present, the discussion on the increase of thermal conductivity at high temperatures for thermal barrier coatings is not comprehensive enough. The present study systematically explored the increased behavior of high temperature thermal conductivity for (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xTix)2O7 high-entropy ceramics. Above 600 ℃, the thermal conductivities of x = 0.1–0.5 components increase with composition and temperature. The significantly reduced emissivity and improved transmittance imply that the reason for the increase of high temperature thermal conductivity is the remarkable improvement of photon thermal conductivity. Importantly, this study provides a route to explore the different behavior of high temperature thermal conductivity by investigating phonon, photon and electronic heat conduction behaviors, which is also conducive to design and develop advanced thermal barrier coatings with low thermal conductivity.

Ultrafast generation and detection of coherent acoustic phonons in SnS0.91Se0.09

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

Phononic thermal conduction and thermal regulation in low-dimensional micro-nano scale systems: Nonequilibrium statistical physics problems from chip heat dissipation

“Heat death”, namely, overheating, which will deteriorate the function of chips and eventually burn the device and has become an obstacle in the roadmap of the semiconductor industry. Therefore, heat dissipation becomes a key issue in further developing semiconductor. Heat conduction in chips encompasses the intricate dynamics of phonon conduction within one-dimensional, two-dimensional materials, as well as the intricate phonon transport through interfaces. In this paper, the research progress of the complexities of phonon transport on a nano and nanoscale in recent three years, especially the size dependent phonon thermal transport and the relationship between anomalous heat conduction and anomalous diffusion are summarized. Further discussed in this paper is the fundamental question within non-equilibrium statistical physics, particularly the necessary and sufficient condition for a given Hamiltonian whose macroscopic transport behavior obeys Fourier’s law. On the other hand, the methods of engineering the thermal conduction, encompassing nanophononic crystals, nanometamaterials, interfacial phenomena, and phonon condensation are also introduced. In order to comprehensively understand the phononic thermal conduction, a succinct overview of phonon heat transport phenomena, spanning from thermal quantization and the phonon Hall effect to the chiral phonons and their intricate interactions with other carriers is presented. Finally, the challenges and opportunities, and the potential application of phonons in quantum information are also discussed.

A physics rule to design aperiodic width-modulated waveguides for minimum phonon transmission with Bayesian optimization.

Aperiodic nano-waveguides (nWVGs) and superlattices (SLs) limit phonon transmission and heat conduction much more efficiently than periodic ones. They could block parasitic heat conduction that restricts heat management and energy conversion at the nanoscale. Aperiodicity can be realized in multiple ways and with variable degrees of complexity. Machine learning optimization of hetero-SLs for minimum coherent phonon conduction showed optimal aperiodicity for moderate disorder against physics intuition. Here, we report on optimal aperiodicity in width-modulated nWVGS for maximum disorder as expected by physics. Optimizing aperiodic geometry modulation is particularly challenging due to the enormous possible configurations. We set up a feasible optimization problem removing unnecessary complexity and we demonstrate efficient Bayesian optimization. Our results confirm the predicted physics rule that minimum thermal conductance occurs for the most disordered arrays of modulation units; the degree of disorder being quantified by the number of non-identical modulation units. Our work opens a route to design geometrical aperiodicity and control transmission across metamaterials.

Extolling thermoelectric properties of incommensurate (IC) TlBiSe2 polycrystals prepared by melt temperature oscillation method

The high-ordered crystal structure of a thermoelectric material does not delimit the phonon conductivity but shows improved electrical properties. Disordered crystal structures act as scattering centers for phonon, but at the expense of the electrical properties of the material. Hence, a diminished thermal (phonon) conductivity and improved electronic conductivity can be obtained from an incommensurate (IC) phase structure. This paper reports that TlBiSe2 polycrystal material exhibits incommensurate (IC) phase (disordered structure). The ‘Tl+’ connecting chains and the disordered structure inherent in the IC phase enhance the TE properties in TlBiSe2. The crystalline phase was confirmed for TlBiSe2 by X-ray diffraction (XRD). These polycrystals have shown their characteristic Raman modes and micro flake-like morphology, as observed by Raman spectroscopy and field emission scanning electron microscopy (FESEM). X-ray photoelectron spectroscopy confirmed the IC phase formation by observing the characteristic binding energy shift of ‘Tl’ and ‘Se’ states. The parallel plane symmetry break in the IC phase has resulted in a hall carrier concentration (n) of 3.49 × 1020 cm−3 and mobility (μ) of 8.27 m2V−1s−1. Further, Seebeck coefficient showed 80 μV/K; the Wiedemann Franz law calculation reserved a lower electronic thermal conductivity of 0.003381 Wm−1K−1 and a power factor of 2.956 μW/m K2 has been attained.

Phononic heat conductance of gold atomic contacts: Coherent versus incoherent transport

We present here a theoretical method to determine the phononic contribution to the thermal conductance of nanoscale systems in the phase-coherent regime. Our approach makes use of classical molecular dynamics (MD) simulations to calculate the temperature-dependent dynamical matrix, and the phononic heat conductance is subsequently computed within the Landauer-B\"uttiker formalism with the help of nonequilibrium Green's function techniques. Tailored to nanostructures, crucial steps of force constant and heat transport calculations are performed directly in real space. As compared to conventional density functional theory (DFT) approaches, the advantage of our method is two-fold. First, interatomic interactions can be described with the method of choice. Semiempirical potentials may lead to large computational speedups, enabling the study of much larger systems. Second, the method naturally takes into account the temperature dependence of atomic force constants, an aspect that is ignored in typical static DFT-based calculations. We illustrate our method by analyzing the temperature dependence of the phononic thermal conductance of gold (Au) chains with lengths ranging from 1 to 12 atoms. Moreover, in order to evaluate the importance of anharmonic effects in these atomic-scale wires, we compare the phase-coherent approach with nonequilibrium MD (NEMD) simulations. We find that the predictions of the phase-coherent method and the classical NEMD approach largely agree above the Debye temperature for all studied chain lengths, which shows that heat transport is coherent and that our phase-coherent approach is well suited for such nanostructures.

Three-dimensional boron nitride network/polyvinyl alcohol composite hydrogel with solid-liquid interpenetrating heat conduction network for thermal management

• Solid-liquid interpenetrating heat conduction network. • Heat conduction and storage function. • Efficient thermal management materials. Polyvinyl alcohol hydrogels have been used in wearable devices due to their good flexibility and biocompatibility. However, due to the low thermal conductivity ( κ ) of pure hydrogel, its further application in high power devices is limited. To solve this problem, melamine sponge (MS) was used as the skeleton to wrap boron nitride nanosheets (BNNS) through repeated layering assembly, successfully preparing a three-dimensional (3D) boron nitride network (BNNS@MS), and PVA hydrogels were formed in the pores of the network. Due to the existence of the continuous phonon conduction network, the BNNS@MS/PVA exhibited an improved κ . When the content of BNNS is about 6 wt.%, κ of the hydrogel was increased to 1.12 W m −1 K −1 , about two times higher than that of pure hydrogel. The solid heat conduction network and liquid convection network cooperate to achieve good thermal management ability. Combined with its high specific heat capacity, the composites have an important application prospect in the field of wearable flexible electronic thermal management.

Local bistability under microwave heating for spatially mapping disordered superconductors

We theoretically study a strongly disordered superconducting layer heated by near-field microwave radiation from a nanometric metallic tip. The microwaves heat up the quasiparticles, which cool by phonon emission and conduction away from the heated area. Due to a bistability with two stable states of the electron temperature under the tip, the heating can be tuned to induce a submicrometer-sized normal region bounded by a sharp domain wall between high- and low-temperature states. We propose this as a local probe to access different physics from existing methods, for example, to map out inhomogeneous superfluid flow in the layer. The bistability-induced domain wall can significantly improve its spatial resolution.

Superelastic, Highly Conductive, Superhydrophobic, and Powerful Electromagnetic Shielding Hybrid Aerogels Built from Orthogonal Graphene and Boron Nitride Nanoribbons.

Three-dimensional (3D) elastic aerogels enable diverse applications but are usually restricted by their low thermal and electrical transfer efficiency. Here, we demonstrate a strategy for fabricating the highly thermally and electrically conductive aerogels using hybrid carbon/ceramic structural units made of hexagonal boron nitride nanoribbons (BNNRs) with in situ-grown orthogonally structured graphene (OSG). High-aspect-ratio BNNRs are first interconnected into a 3D elastic and thermally conductive skeleton, in which the horizontal graphene layers of OSG provide additional hyperchannels for electron and phonon conduction, and the vertical graphene sheets of OSG greatly improve surface roughness and charge polarization ability of the entire skeleton. The resulting OSG/BNNR hybrid aerogel exhibits very high thermal and electrical conductivity (up to 7.84 W m-1 K-1 and 340 S m-1, respectively) at a low density of 45.8 mg cm-3, which should prove to be vastly advantageous as compared to the reported carbonic and/or ceramic aerogels. Moreover, the hybrid aerogel possesses integrated properties of wide temperature-invariant superelasticity (from -196 to 600 °C), low-voltage-driven Joule heating (up to 42-134 °C at 1-4 V), strong hydrophobicity (contact angel of up to 156.1°), and powerful broadband electromagnetic interference (EMI) shielding effectiveness (reaching 70.9 dB at 2 mm thickness), all of which can maintain very well under repeated mechanical deformations and long-term immersion in strong acid or alkali solution. Using these extraordinary comprehensive properties, we prove the great potential of OSG/BNNR hybrid aerogel in wearable electronics for regulating body temperature, proofing water and pollution, removing ice, and protecting human health against EMI.

Mapping phonon hydrodynamic strength in micrometer-scale graphite structures

With the peculiar collective transport behaviors and potential applications in thermal management, phonon hydrodynamics at elevated temperatures draws increasing attention in host materials, such as graphite. We map the strength of steady-state phonon hydrodynamic flow in 12C purified graphite micro-structures with finite length and width in a broad range of sizes and temperatures. Our theoretical modeling demonstrates that hydrodynamic phonon conduction is largely strengthened and shifts to lower temperature ranges with increasing width from a few micro-meters to 10 μm. The present work provides an insight into phonon hydrodynamics in finite-sized graphitic materials and guides its experimental observation.

Effects of high-order anharmonicity on anomalous lattice dynamics and thermal transport in fully filled skutterudite YbFe4Sb12

Accurately describing the lattice dynamics and microscopic mechanism of thermal transport in materials with low-lying flat phonon modes remains an outstanding challenge due to the intrinsic strong anharmonicity. In this paper, we investigate the lattice dynamics and thermal transport in skutterudite ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12}$ using a state-of-the-art first-principles-based anharmonic phonon renormalization technique and a unified theory of lattice thermal transport. In contrast to the previous phenomenological models that introduce additional resonant scattering terms or hopping channels, we show that the unusual total lattice thermal conductivity in ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12}$ can be accurately predicted by considering anharmonic phonon renormalization and coherence contributions from the off-diagonal terms of heat flux operators. Both the cubic and quartic anharmonicities are essential for precisely predicting the significant shift in phonon energies. Specifically, the anharmonicity-induced phonon stiffening of the low-lying flat modes significantly enhances the thermal conductivity of particlelike phonons, e.g., by up to a factor of 1.6 at 300 K, by suppressing the cubic coupling strength and altering the scattering phase space, resulting in much-improved agreement with experiments. By further including the coherence contributions, the predicted total thermal conductivity increases by \ensuremath{\sim}22% throughout the entire temperature range, reproducing well the experimental values in both magnitude and temperature dependence. In this paper, we highlight the strong impact of higher-order anharmonicity on lattice dynamics and thermal transport in the filled skutterudite ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12}$. The insights gained in this paper will be helpful for manipulating the thermal properties of skutterudites and potentially other complex materials with strong anharmonicity, which can improve their performance in applications such as thermoelectrics, ferroelectrics, and photovoltaics.

Silver nanoparticles bridging segregated hexagonal boron nitride networks for enhancing the thermal conductivity of polystyrene composites

The segregated heat conductive networks constructed from two-dimensional (2D) hexagonal boron nitride (hBN) are promising in improve the thermal conductivity of polymer-based thermal management materials (TMMs), however, the high interfacial thermal resistance between adjacent hBN sheets remains challenging. Hence, high-performance polystyrene (PS)-based composites with continuous segregated hBN@AgNP networks were designed and prepared by hot-pressing method. Benefited from the modification and bridge effect of AgNP, the interconnected hBN exhibited significantly reduced contact thermal resistance, which provide the effective pathways for phonon conduction. The as-fabricated PS/hBN@AgNP composite exhibited high thermal conductivity (1.17 and 1.53 W/mK, 30 wt%) corresponding to enhancements of 738.4% and 989.9% compared with neat PS, significantly higher than random PS/hBN and composites without AgNP in the segregated structure. Therefore, this work provides a strategy for the preparation of high-performance polymer-based TMMs by reducing the interfacial thermal resistance in interconnected filler network.

Three-dimensional boron nitride reinforced thermal conductive composites with high elasticity

Polymer-based thermally conductive composites have good interfacial contact, processability and stability. However, because of the low thermal conductivity of polymer materials, usually, a larger percentage of filler is required to achieve high thermal conductivity, which leads to a drop in the resilience of the composite. To solve this problem, a melamine-formaldehyde resin sponge (MS) was used as a skeleton to wrap boron nitride nanosheets (BNNS) through repeated layering assembly, successfully preparing a three-dimensional (3D) boron nitride network (BNNS@MS). The mixture of poly(vinylidene fluoride) and BNNS (PVDF/BNNS) was formed in the pores of the 3D network of BNNS@MS. Due to the existence of the continuous phonon conduction network, the BNNS@MS/PVDF exhibits good thermal conductivity and high elastic deformation (rebound at 90% compression). When the content of BNNS is about 5 wt%, the thermal conductivity of the BNNS@MS/PVDF is increased to 1.43 ± 0.2 W/mK, 51.1% higher than that of PVDF/BNNS. The solid heat conduction network and polymer interplay to achieve good thermal management ability, and the composites have an important application prospect in the field of wearable flexible electronic thermal management.

Synthesis of a monolayer fullerene network.

Two-dimensional (2D) carbon materials, such as graphene, have attracted particular attention owing to the exceptional carrier transport characteristics that arise from the unique π-electron system in their conjugated carbon network structure1-4. To complement zero-bandgap graphene, material scientists have devoted considerable effort to identifying 2D carbon materials5-8. However, it is a challenge to prepare large-sized single-crystal 2D carbon materials with moderate bandgaps5,9. Here we prepare a single-crystal 2D carbon material, namely monolayer quasi-hexagonal-phase fullerene (C60), with a large size via an interlayer bonding cleavage strategy. In this monolayer polymeric C60, cluster cages of C60 are covalently bonded with each other in a plane, forming a regular topology that is distinct from that in conventional 2D materials. Monolayer polymeric C60 exhibits high crystallinity and good thermodynamic stability, and the electronic band structure measurement reveals a transport bandgap of about 1.6 electronvolts. Furthermore, an asymmetric lattice structure endows monolayer polymeric C60 with notable in-plane anisotropic properties, including anisotropic phonon modes and conductivity. This 2D carbon material with a moderate bandgap and unique topological structure offers an interesting platform for potential application in 2D electronic devices.

Suppressed phonon conduction by geometrically induced evolution of transport characteristics from Brownian motion into Lévy flight

Despite extensive research on quasi-ballistic phonon transport, anomalous phonon transport is still observed in numerous nanostructures. Herein, we investigate the transport characteristics of two sets of samples: straight beams and nanoladders comprising two straight beams orthogonally connected with bridges. A combination of experiments and analysis with a Boltzmann transport model suggests that the boundary scattering within the bridges considerably dictates the distribution of phonon mean free paths, despite its negligible contribution to the net heat flux. Statistical analysis of those boundary scatterings shows that phonons with large axial angles are filtered into bridges, creating dead spaces in the line-of-sight channels. Such redistribution induces Lévy walk conduction along the line-of-sight channels, causing the remaining phonons within the bridges to exhibit Brownian motion. Phonon conduction in the nanoladders is suppressed below that of the straight beams with equivalent cross-sectional areas due to trapped phonons within the bridges. Our work reveals the origin of unusual thermal conductivity suppression at the nanoscale, suggesting a method to modulate phonon conduction via systematic nanostructuring.

Fast-thermoresponsive carboxylated carbon nanotube/chitosan aerogels with switchable wettability for oil/water separation

The use of aerogels to selectively recover oil from oily wastewater is effective but challenging. In this study, a new carboxylated carbon nanotube/chitosan aerogel (CCNT/CA) with switchable wettability was developed as a smart adsorbent for fast oil absorption and oil recovery. Vinyltrimethoxysilane and thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) was grafted onto the surface of the CCNT/CA skeleton, and the resulting smart aerogel (PNI-Si@CCNT/CA) exhibited temperature responsiveness. PNI-Si@CCNT/CA exhibited an excellent reversible conversion between hydrophilicity and hydrophobicity when the temperature was changed to below or above the lower critical solution temperature (LCST) of PNIPAAm (~32 °C). Most importantly, CCNT significantly increased the oil absorption capacity, improved the mechanical properties, accelerated phonon conduction, enhanced thermal conductivity (80.57 mW m−1 K−1), improved the temperature response rate, shortened the oil desorption time (15 min), and improved the oil/water separation efficiency of PNI-Si@CCNT/CA because a strong interface interaction occurred between CCNT and chitosan. Moreover, PNI-Si@CCNT/CA absorbed oil at 45 °C and released the absorbed oil at 25 °C. It maintained its good adsorption performance after 15 cycles, and this was ascribed to its excellent mechanical properties and stable structure.